[PATCH] D129531: [clang][C++20] P0960R3 and P1975R0: Allow initializing aggregates from a parenthesized list of values

[Ilya Biryukov via Phabricator via cfe-commits]

ilya-biryukov added inline comments.


================
Comment at: clang/lib/CodeGen/CGExprAgg.cpp:581
+  Expr *filler = nullptr;
+  if (auto *ILE = dyn_cast<InitListExpr>(ExprToVisit))
+    filler = ILE->getArrayFiller();
----------------
ilya-biryukov wrote:
> ayzhao wrote:
> > ilya-biryukov wrote:
> > > - Should we have a filler for `CXXParenInitListExpr` too?
> > >   It seems like an important optimization and could have large effect on compile times if we don't 
> > > 
> > > - Same question for semantic and syntactic-form (similar to `InitListExpr`): should we have it here?
> > >   I am not sure if it's semantically required (probably not?), but that's definitely something that `clang-tidy` and other source tools will rely on.
> > > 
> > > We should probably share the code there, I suggest moving it to a shared base class and using it where appropriate instead of the derived classes.
> > > Should we have a filler for CXXParenInitListExpr too? It seems like an important optimization and could have large effect on compile times if we don't
> > 
> > This feels like premature optimization - presumably, wouldn't this only be an issue with extraordinarily large (say, O(1000)) arrays?
> > 
> > > Same question for semantic and syntactic-form (similar to InitListExpr): should we have it here? I am not sure if it's semantically required (probably not?), but that's definitely something that clang-tidy and other source tools will rely on
> > 
> > IIRC this doesn't apply to paren list aggregate expressions, as the syntatic form would be the enclosing `ParenListExpr`.
> > This feels like premature optimization - presumably, wouldn't this only be an issue with extraordinarily large (say, O(1000)) arrays?
> Yes, this should only happen with large arrays. Normally I would agree, but it's surprising that changing `{}` to `()` in the compiler would lead to performance degradation.
> In that sense, this premature optimization is already implemented, we are rather degrading performance for a different syntax to do the same thing.
> 
> Although we could also land without it, but in that case could you open a GH issue and add a FIXME to track the implementation of this particular optimization?
> This should increase the chances of users finding the root cause of the issue if they happen to hit it.
> 
> > IIRC this doesn't apply to paren list aggregate expressions, as the syntatic form would be the enclosing ParenListExpr.
> Do we even have the enclosing `ParenListExpr`? If I dump the AST with `clang -fsyntax-only -Xclang=-ast-dump ...`  for the following code:
> ```
> struct pair { int a; int b = 2; };
> int main() {
>   pair(1); pair p(1);
>   pair b{1}; pair{1};
> }
> ```
> 
> I get 
> ```
> `-FunctionDecl 0x557d79717e98 <line:2:1, line:5:1> line:2:5 main 'int ()'
>   `-CompoundStmt 0x557d797369d0 <col:12, line:5:1>
>     |-CXXFunctionalCastExpr 0x557d79718528 <line:3:3, col:9> 'pair':'pair' functional cast to pair <NoOp>
>     | `-CXXParenListInitExpr 0x557d79718500 <col:3> 'pair':'pair'
>     |   |-IntegerLiteral 0x557d79718010 <col:8> 'int' 1
>     |   `-IntegerLiteral 0x557d79717e18 <line:1:30> 'int' 2
>     |-DeclStmt 0x557d79718650 <line:3:12, col:21>
>     | `-VarDecl 0x557d79718568 <col:12, col:17> col:17 p 'pair':'pair' parenlistinit
>     |   `-CXXParenListInitExpr 0x557d79718610 <col:17> 'pair':'pair'
>     |     |-IntegerLiteral 0x557d797185d0 <col:19> 'int' 1
>     |     `-IntegerLiteral 0x557d79717e18 <line:1:30> 'int' 2
>     |-DeclStmt 0x557d797187d8 <line:4:3, col:12>
>     | `-VarDecl 0x557d79718680 <col:3, col:11> col:8 b 'pair':'pair' listinit
>     |   `-InitListExpr 0x557d79718750 <col:9, col:11> 'pair':'pair'
>     |     |-IntegerLiteral 0x557d797186e8 <col:10> 'int' 1
>     |     `-CXXDefaultInitExpr 0x557d797187a0 <col:11> 'int'
>     `-CXXFunctionalCastExpr 0x557d797369a8 <col:14, col:20> 'pair':'pair' functional cast to pair <NoOp>
>       `-InitListExpr 0x557d79718868 <col:18, col:20> 'pair':'pair'
>         |-IntegerLiteral 0x557d79718800 <col:19> 'int' 1
>         `-CXXDefaultInitExpr 0x557d797188b8 <col:20> 'int'
> ```
> It feels like the `ParentListExpr` is replaced during semantic analysis and there is no way to get it back. I also tried running `clang-query` and trying to `match parenListExpr()` and go 0 results. Is it just missing in the AST dump and I run `clang-query` incorrectly or do we actually not have the syntactic form of this expression after all?
Another important thing to address from the dump: notice how braced initialization creates `CXXDefaultInitExpr` and `CXXParenListInitExpr` copies the default argument expression directly. It's really important to use the former form, here's the example that currently breaks:


```
#include <iostream>

struct func_init { int some_int; const char* fn = __builtin_FUNCTION(); };

int main() {
  func_init a(10);
  std::cout << "From paren init:" << a.fn << std::endl;

  func_init b{10};
  std::cout << "From braced init: " << b.fn << std::endl;
}
```

The program above is expected to report `main` for both cases, but instead we get:
```
>From paren init:
>From braced init: main
```


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https://reviews.llvm.org/D129531